我对ORACLE数据锁的一点体会，我对oracle

我对ORACLE数据锁的一点体会，我对oracle

该贴的部分内容是针对帖中带的附件（一位大侠关于锁的理解），提的理解/疑问，
大家在读此贴前，请先阅读该文。
先说说各类型的锁：共享锁，排它说，共享排它锁（对表定义共享，对表操作的记录排它）。
1、我对文章中意向锁的理解，就是：对表的记录进行操作之前，先对表定义（包括表结构、约束等） 
 加了共享锁，这是为了避免对表的DDL操作。 
 比如：当你往TAB1插入一条记录时，该表的一个字段COL8是允许为空的，插入这条记录的该字段 
 的值是空的，此时，若不对该表定义加共享锁，则另外一SESSION对TAB1.COL8加非空约束， 
 那该表结构修改就与插入的记录发生冲突了。所以，当执行DML操作是，必定对操作的表加DDL 
 共享锁。  这是我理解的意向锁。 2、文章中提及，记录行是无共享锁。我对此观点表示不同：当插入/修改子表时，对应的父表的主键 
 记录应该被加共享锁，因为此时要保证父键的存在。经测试，当插入/修改/删除子表时，父表 
 确实被加了SS锁，此时可以对父表的主键做插入动作，但不允许做修改/删除。这符合逻辑。因为 
 插入操作并不影响主外键的关系。但删除/修改则可能。 
3、SHARE VS ROW SHARE有什么区别？ 
 我猜想：ROW SHARE是（级别=2）对表加表定义共享锁，SHARE（级别=4）是对表定义及表“所有”的 
 记录加共享锁。但这样就有问题：1、SELECT FOR UPDATE产生TM=2、对查询出来的记录产生TX=6的 
 锁，那这样应该与：表“所有”的记录加共享锁 相冲突了？但实际并不是，实际上，当对表加SHARE 
 锁时，还是可以该表执行 SELECT FOR UPDATE，但却不可以执行DML操作！
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